Global warming potential
A metric comparing the integrated radiative effect of equal-mass gas emissions over a chosen period, relative to CO₂.
Example
It allows different greenhouse gases to be expressed in CO₂-equivalent units.
The catch
The value changes with the time horizon and scientific assessment.
In more depth
Global warming potential, explained.
Global warming potential works like an exchange rate between greenhouse gases. Scientists calculate it by summing the extra heat trapped by a pulse of a gas over a set horizon, then dividing by the figure for the same mass of CO₂. A short-lived gas does most of its warming early, so the choice of horizon largely sets its rate.
Methane shows the swing. It stays in the atmosphere for about 12 years, and mass for mass it warms about 80 times as much as CO₂ over 20 years and about 30 times over 100 years (IPCC AR6, UNEP). Most inventories and corporate reports use GWP100, and a carbon dioxide equivalent figure carries that horizon unless the report says otherwise.
The metric has limits. It sums warming effect over time, which is a different quantity from the temperature reached in a given year. Alternative metrics, such as the global temperature change potential, answer other questions and give methane a lower weight over long horizons. Reports should name the IPCC edition behind their values, since the rules for official inventories can lag the newest assessment. A claim that switches horizons between pages, or quotes the larger number without saying so, deserves suspicion.